1. Field of the Invention
This invention is directed to regeneration of optical signals for data communications, in particular though not exclusively optical signals comprising optical return-to-zero (RZ) pulses. It has application to both 2R (re-amplification and re-shaping) and 3R (re-amplification, re-shaping and re-timing) regeneration.
As the capacity of transmission systems increases in response to the increasing demand for communication, the maximum reach of each transmission system is diminished. Regenerators are therefore required at regular intervals along a transmission link in addition to any regenerators associated with network nodes where traffic routing takes place. All-optical wavelength converters with 2R regeneration capabilities, that allow operation at speeds beyond the limits of electronic devices, will be essential in future wavelength division multiplexed (WDM) networks. In particular, of high interest are simple and compact wavelength converters that help to avoid wavelength blocking and ease WDM network management.
2. Technical Background
FIGS. 1 and 2 illustrate a device and method for performing all-optical wavelength conversion and 2R regeneration of multiple data signals comprising a series of optical pulses which does not form part of this invention but is helpful in understanding it. The DISC-type (delayed interference signal-wavelength converter) regenerator shown in FIG. 1 comprises a semiconductor optical amplifier 1 for modulating a continuous wave input with the data signal, and an interferometer 5 having a delay 7 in one arm for causing the modulated continuous wave input to interfere with a delayed copy thereof to regenerate the data signal. The copy of the modulated continuous wave input is delayed by less than one data signal pulse width with respect to the modulated continuous wave input, whereby the pulse width of the regenerated data signal is similar to the pulse width of the input data signal. By reducing the time delay of the copy of the modulated continuous wave input to less than one input optical pulse width, the amplitude of the regenerated data signal is not significantly reduced, and the pulse width of the regenerated data signal can be maintained at close to that of the input data signal. This enables the regenerator to be used in real networks to regenerate broad optical pulses without causing excessive additional broadening of the pulses. This means that several such regenerators may be used to re-amplify and reshape a signal at regular intervals along a transmission path to increase transmission distances between more expensive 3R regenerators.
As each optical pulse of a data signal is transmitted through the semiconductor optical amplifier 1, the relative phase of the continuous wave input transmitted through the amplifier is temporarily altered until the amplifier recovers. Thus, the relative phase at the output from the undelayed arm of the interferometer changes progressively leading to a progressive increase in the non-inverting output level 9 from the interferometer and a corresponding decrease in the inverting output level 11 (depending on the relative phase delay between the two arms) for the duration of the delay. The relative phase at the output from the delayed arm of the interferometer changes progressively after the duration of the delay, reducing the non-inverting output level from the interferometer until the delayed and undelayed parts cancel each other completely. Because the change in phase is progressive, the pulse output by the interferometer is wider than the delay time. If consecutive pulses are transmitted at a faster rate than the recovery rate of the amplifier, the amplifier will not fully recover after each pulse, leading to a reduced phase differential on the modulated continuous wave input corresponding to the difference between the presence and absence of a data pulse.
Optimum operation of a regenerator based on cross-phase modulation in SOAs and an interferometer requires the phase differences between the two arms to be such that the transmission is at a minimum when a data ‘zero’ level (no pulse) is received and at a maximum when a data ‘one’ level (pulse) is received (or vice versa for inverting operation). Therefore, ideally, an input pulse should cause a phase change of π as illustrated in FIG. 2. At high bit rates, where the period is less than the recovery time, it may not be possible to obtain a full π phase change. In this case, adjusting the phase offset in one arm will allow either the ‘zero’ level or the ‘one’ level to be at a stationary point of the interferometer transfer characteristic, but not both. Thus, if the phase offset is adjusted such that the ‘one’ level is at a stationary point of the interferometer transfer characteristic, the non-inverting output levels 9 from the interferometer may consistently achieve their maximum output power corresponding to the data pulses, while returning to a raised level between consecutive pulses because the phase shift induced by the pulses will not have had time to reduce to zero before arrival of the next pulse. Such partial switching allows noise on the ‘zero’ level at the input to be transferred to the output of the interferometer. It is therefore clear that data signal rates higher than the recovery rate of the amplifier lead to reduced signal to noise ratio of the regenerated data signal. There is therefore a need to provide improved optical signal regeneration at higher data rates.